The present invention relates to electrochromic devices (hereinafter referred to as ECDs) and, more particularly to viologen compounds and mixed electrolyte systems of aqueous solutions of electrochromic viologen compounds used in ECDs which have a long life and short response time.
Specialized missiles require an "iris" to rapidly attenuate light intensity. Over the lifetime of the missile, this can easily involve over 100,000 cycles. Mechanical devices are subject to failure resulting from G-force stresses and the wear and corrosion of moving parts. Furthermore, mechanical devices add to the weight and are subject to the space limitations of the missile. A self-contained electrochemical system would have no moving parts and would be limited only by the stability of the materials and the reversibility of the electrochemical reactions. An electrochemical light attenuation device could be simply constructed in the form of a thin disk; thus, yielding a savings of space, weight and cost.
In missile applications such a device requires faster switching time (1 Hz) and a smaller size (5 cm.sup.2) than required for commercial applications. Long-term stability (5-10 years) and good reversibility (100,000 cycles) are also important characteristics for missile applications. Another requirement is that light transmitted by this device should have good optical quality.
Solutions of viologen compounds are used for the reversible deposition of highly colored films at an electrode surface. The electrochromic effect requires the reduction of the colorless aqueous solution of the dicationic viologen salt to a highly colored insoluble radical ion salt that is deposited as a film on the cathode. The film will remain fixed on the electrode surface but may be rapidly erased by reversing the current which oxidizes the film back to soluble species.
The major advantages of viologen electrochromic systems include fast response time, a low operating voltage, and the complete removal of the film upon oxidation to give maximum light transmittance. The electrode process for film formation or removal require only a few tenths of a second for small active areas.
The major problem with viologen systems is that an aging or crystallization effect limits the number of reversible cycles that can be obtained. After repeated cycling, films gradually assume a greater degree of optical anisotropy, consistent with the reorientation of the molecules to a higher degree of ordering. The formation of polycrystalline conglomerates has also been noted. This aging process results in a drop in erase efficiency.
The prior art has focused on the commercially available dimethyl and diheptyl viologens, and has primarily used aqueous solutions of potassium bromide. The molecular structure of the viologen and the nature of the anion are known to affect the stability of the radical ion salt film. One study examined replacing the commonly used bromide anion with dihydrogen phosphate. R. J. Jasinski, J. Electrochem. Soc., 124, 637 (1977). Another study examined the influence of molecular structure on the stability of the radical ion salt film, J. A. Barltrop and A. C. Jackson, J. Chem. Soc, Perkin Trans. II, 367 (1984); however, the detrimental effects of aging have not been significantly reduced. Consequently, the highest number of reversible cycles reported in the literature for viologen materials is 20,000 cycles.
It is therefore one object of this invention to provide an electrochromic system capable of completing over 20,000 reversible cycles.
Another object of this invention is to provide an electrochromic system which can be used in a small, lightweight electrochromic device.
A further object of this invention is to provide an electrochromic system which is capable of supporting fast, repeatable cycles.
A still further object of this invention is to provide novel viologen compounds.
It is still another object of this invention to provide novel viologen compounds which can be used in the above electrochromic system.
It is yet another object of this invention to provide novel electrolyte systems for use in electrochromic systems.
It is still a further object of this invention to provide novel solvent systems-for use in electrochromic systems.